Furnaces are used in many different industrial applications to heat and/or melt a charge such as steel. Furnaces may be generally categorized as being of two types, the indirectly-fired type and the directly-fired type. Indirectly-fired furnaces employ radiant tubes, through which hot fluid passes, or electrical resistance coils, to provide heat into the furnace which is then used to heat the charge. Directly-fired furnaces employ one or more burners to combust fuel and oxidant to provide heat into the furnace for use to heat the charge.
Directly-fired furnaces are preferred over indirectly-fired furnaces because an indirectly-fired furnace is generally more expensive to construct and generally also has higher maintenance costs. Moreover, heat transfer efficiency is generally lower in indirectly-fired furnaces than in directly-fired furnaces. However, if product quality is a significant issue, it may be necessary to employ an indirectly-fired furnace despite its disadvantages. This is because in a directly-fired furnace, combustion gases are injected into the furnace atmosphere from the burner or burners which provide the heat to operate the furnace. Combustion gases include water vapor, carbon dioxide and oxygen and may also include nitrogen, uncombusted or incompletely combusted fuel, and other gases. One or more of the combustion gases may have a deleterious effect upon the furnace charge when such gas or gases contacts the charge.
It is preferable for fuel efficiency as well as for environmental reasons to employ pure oxygen or an oxygen-enriched fluid rather than air as the oxidant with a burner in a directly-fired furnace. However, oxygen or oxygen-enriched fluid generally requires a higher combustion mixture injection velocity from the burner in order to avoid hot spots and uneven combustion. Such higher velocity results in greater mixing of the combustion mixture but also causes more of the combustion gases to come in contact with the charge.
Accordingly, it is an object of this invention to provide a direct-fired furnace system which will enable effective heating and/or melting of the furnace charge while reducing the deleterious effects resulting from contact of the furnace charge with combustion gas or gases.